Pure Substances & Mixtures

Question 1

A 2.0 g sample of a white solid is heated. The pure substance is known to melt sharply at 150 °C. Instead, the sample begins to melt at 145 °C and finishes melting at 160 °C. Which statement best explains this observation?

Accepted Answer

The sample contains impurities that lower the melting point and cause it to occur over a range of temperatures.

Answer

The sample is a pure substance, but the thermometer reading is inaccurate, making it appear to melt over a range.

Answer

The sample is undergoing a chemical reaction that produces a gas, so the observed temperature range reflects decomposition rather than melting.

Answer

The sample is an alloy that melts at a single temperature higher than the pure substance’s melting point.

Question 2

A pure substance melts at a sharp, fixed temperature, but a mixture of two salts melts over a range of temperatures. Which explanation best accounts for this behavior?

Accepted Answer

Each component of the mixture melts at its own characteristic temperature, so the mixture melts over a range of temperatures.

Answer

The components react to form a new compound that has one sharp melting point.

Answer

The mixture’s boiling point is higher than the boiling points of the pure substances, causing the observed melting behavior.

Answer

The impurity causes the mixture to melt at a single lower temperature than either pure component.

Question 3

Pure substance A has a sharp melting point of 150 °C, and pure substance B has a sharp melting point of 200 °C. A sample that contains 40 % A and 60 % B is observed to begin melting at 150 °C and to be completely melted only when the temperature reaches 200 °C. Which statement best explains this melting behavior?

Accepted Answer

Each component in the mixture melts at its own characteristic temperature, so the mixture starts to melt when the lower‑melting component (A) begins to melt and finishes melting when the higher‑melting component (B) has melted.

Answer

The two substances react to form a new compound that has a single melting point intermediate between 150 °C and 200 °C.

Answer

The mixture exhibits eutectic behavior, giving it a fixed melting point at 175 °C.

Answer

Impurities in the mixture lower the melting point of A and raise the melting point of B, causing the melting to occur over a temperature range.

Try three questions

A 2.0 g sample of a white solid is heated. The pure substance is known to melt sharply at 150 °C. Instead, the sample begins to melt at 145 °C and finishes melting at 160 °C. Which statement best explains this observation?

A pure substance melts at a sharp, fixed temperature, but a mixture of two salts melts over a range of temperatures. Which explanation best accounts for this behavior?

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